Is the growth of temperate forest trees enhanced along an ambient nitrogen deposition gradient?

The extent to which atmospheric N deposition is enhancing primary production and CO2sequestration along the ambient N deposition gradients found within many regional temperate forest ecosystems remains unknown. We used tree diameter measurements from 1984 and 2004, allometric equations, and estimates of wet N deposition from 32 permanent plots located along an ambient N deposition gradient in the Adirondack Park, New York, USA, to determine the effects of N deposition on the basal area and woody biomass increments (BAI and WBI, respectively) of individual stems from all the major tree species. Nitrogen deposition had either a neutral or positive effect on BAI and WBI, with the positive effects especially apparent within the smaller size classes of several species. The nature of these growth responses suggests that other co-varying factors (e.g., temperature, tropospheric ozone, soil acidification) may be partially counteracting the species-dependent fertilization effect of N deposition that was suggested by recent foliar N data across this gradient. Nevertheless, in documenting a fertilization effect from chronic, low-level, ambient rates of N deposition, this study underscores the need for more research on how N deposition is affecting rates of primary production, CO2sequestration, and even vegetation dynamics in many forests worldwide

Effects of nutrient addition and soil drainage on germination of N-fixing and non-N-fixing tropical dry forest tree species

To develop generalised predictions regarding the effects of atmospheric nitrogen (N) and phosphorus (P) deposition on vegetation communities, it is necessary to account for the impacts of increased nutrient availability on the early life history stages of plants. Additionally, it is important to determine if these responses (a) differ between plant functional groups and (b) are modulated by soil drainage, which may affect the persistence of added nutrients. We experimentally assessed seed germination responses (germination proportion and germination energy, i.e. time to germination) of commonly occurring N-fixing and non-N-fixing tropical dry forest tree species found in India to simulated N and P deposition in well-drained soils, as well as soils with impeded drainage. When soils were not allowed to drain, germination proportion declined with nutrient addition, while germination energy remained unchanged. Stronger declines in germination proportion were observed for N-fixing species. In free-draining soils, nutrient addition did not affect germination proportion in either functional group. However, we detected a trend of delayed germination with nutrient addition, especially in N-fixers. Our results suggest that nutrient deposition can lead to potential shifts in functional dominance and tree community composition of tropical dry forests in the long term through its effects on early life stages of trees, although the mechanisms underlying the observed germination responses remain unclear. Further, such effects are likely to be spatially variable across the geographic range in which tropical dry forests occur depending on soil drainage properties

Continued Slow Decay of the Residual Plasma Viremia Level in HIV-1 – Infected Adults Receiving Long-term Antiretroviral Therapy

We measured plasma human immunodeficiency virus type 1 (HIV-1) RNA levels by means of single-copy assay in 334 participants receiving virologically suppressive antiretroviral therapy (ART). A residual viremia load of ≥1 copy/mL after 4 years of ART was predicted by a higher pre-ART HIV-1 RNA level, higher CD8+ T-cell count during treatment, and a lower ratio of CD4+ T cells to CD8+ T cells during treatment but not by initial ART regimen. In a longitudinal subset of 64 individuals, continued decay of the plasma HIV-1 RNA level was observed, with an average annual decrease of 6% and an estimated half-life of 11.5 years. In contrast to prior reports, the persistent viremia level continues to slowly decline during years 4–12 of suppressive ART

Patterns in Soil-Vegetation-Atmosphere Systems: Monitoring, Modeling, and Data Assimilation

In this special issue, we present recent scientific work that analyzes the role of patterns in soil-vegetation-atmosphere (SVA) systems over a wide range of scales ranging from the pore scale up to mesoscale catchments. Specific attention is given to the development of novel data assimilation methods, noninvasive measurement techniques that allow mapping spatial patterns of state variables and fluxes, and two-way coupling of models in a scale-consistent way. "Patterns in Soil-Vegetation-Atmosphere Systems" is also the research topic of a collaborative research center (TR32) between the universities of Aachen, Bonn, and Cologne and the Forschungszentrum Julich. In this center, which is funded by the Deutsche Forschungsgemeinschaft, on the basis of an international evaluation, scientists covering a broad range of earth science disciplines are working together. During June 11-12, 2010 the center organized its first international workshop in Aachen. The contributions presented in this special issue of Vadose Zone Journal include contributions from the collaborative research center and external contributions, both from Germany and worldwide

Decadal-scale comparisons of vegetation and soil properties in forests of the Adirondack Mountains, NY

In 2004, trees were remeasured in permanent vegetation plots that had been established in spruce-fir, northern hardwood, and pine-dominated stands in 1984 in the Adirondack Park, NY. Over the 20-yr interval, live basal area (BA) in red spruce/balsam fir-dominated plots decreased by 15% and live red spruce BA decreased by 42% (P \u3c 0.05). Excluding American beech, live BA in the northern hardwoods decreased significantly over the 20-yr interval. Further, using the same growth measurements and the ambient N deposition gradient in the Adirondack Mountains, the data indicated that N deposition had either a neutral or positive effect on basal area growth, with the positive effects especially apparent within the smaller size classes of several species. The nature of change in vegetation measured in these plots suggested that other co-varying factors (e.g., temperature, tropospheric ozone, soil acidification) may be partially counteracting the species-dependent fertilization effect of N deposition and that northern hardwood plots were not aggrading over the 20-yr interval. In 2005/6, soils were also sampled and analyzed for C, N, soil organic matter (SOM), and HCl-extractable Ca. Those results were compared to data collected in 1932 and 1984 to determine spatial patterns and temporal trends in soil chemistry. As measured in 2005/6, average C (24 kg m-2) and N (1.1 kg m-2) contents of the combined organic and mineral horizons were high for northeastern U.S. forests. Contrary to expectations, atmospheric inputs of N deposited over the past 50 years only accounted for a small portion of soil N, and the differences in N among the plots along the N deposition gradient. There was a significant increase in SUM content between 1932 and 2005/6, which amounted to an increase in soil C content of about 0.5 Mg C ha-1yr-1 at 16 sites that had a history of agriculture and/or fire, indicating that at least some Adirondack forest soils were accumulating C during the 20th century. Further, between 1932 and 2005/6, median HCl-extractable Ca amounts decreased significantly (-79%) in organic horizons and in the whole-profile (organic + mineral horizons; -64%), and the overall rate of whole-profile Ca depletion was 13.6 kg ha-1yr-1 for the pooled data over the 74-yr interval